In stringed musical instruments such as guitars and bass guitars, a main component is the neck that provides or supports a fretboard or fretless fingerboard. The neck is typically made from wood and is ordinarily supplied initially as being nominally flat along its length, free of neck curvature. For purposes of the present disclosure, “neck length” refers to only that portion of the neck that is associated with the fretboard/fingerboard, and is not intended to include any substantial portion of the neck located within the body of the instrument, e.g. as in thru-neck type instrument construction. Even a newly fabricated neck may already have some amount of inherent curvature, concave or convex, and such bowing or arching is not always symmetrical over the neck length.
When the instrument is strung and tuned, the high tension in the strings, in the order of one or two hundred pounds, sets up a strong continuous stress load that tends to bow the neck concavely. Depending on the strength of a particular neck, string tension often introduces some amount of concave curvature that results in a non-uniform increase in the string-to-fret separation, known as “high action”, making the instrument difficult to play by placing excessive demands on fingering technique.
In initial setup or refurbishing of a stringed musical instrument, a luthier or technician typically uses all available adjustment capability to achieve the desired string-to-fret spacing. In the absence of truss tension adjustment, the two main adjustments available are the bridge and nut heights. Many instruments are provided with truss rods with tension adjustment that can reduce neck curvature only if the curvature is concave, and their effect is spread over the full length of the neck.
In the art of the luthier, even a substantially flat neck with uniform low string spacing throughout the neck length would be considered no more than a marginally acceptable compromise. Ideally the luthier would prefer some slight concave curvature, known as “relief”, over a lower pitched section of the neck and a straighter profile over a higher pitched section. This represents a fortuitous circumstance that rarely occurs in manufacture and setup, and cannot normally be accomplished by adjustment in trussed necks of known art, thus requiring laborious “relief” dressing of particular regions of the fretboard by a skilled luthier. Conventional tensioned truss rods or even the relatively rare truss types that can be alternatively placed in compression for adjustment regarding curvature, are all subject to the inherent limitation of known art: their effect is spread over the full length of the neck.
Stringed musical instrument owners, manufacturers, luthiers and repair technicians all face a random variety of variables in instrument neck curvature. Some new necks may already exhibit some amount of curvature: concave or convex. Especially in absence of truss reinforcement, and even with a tension truss, a selection process may be required in quality control wherein necks found to have excessive concave or convex curvature may have to be discarded and replaced. Hopefully an initially straight neck might be strong enough to resist bending and thus remain playable, while a neck with slight convex curvature could even tend to straighten and thus could improve in uniformity and playability after one or more service refurbishments including bridge height adjustments, over a period of time. More typically, undesired neck curvature is likely to worsen with time, and could end up becoming even less correctable with existing truss construction of known art. Asymmetrical curvature affecting only a section of the neck length cannot be adjustably corrected by any truss system in the field, and instead may require extensive dressing of the crowns of the frets by a skilled luthier.